Plant Hardiness

Plant Hardiness

Hardiness is a plant’s ability to thrive in a landscape. To choose the right plant for a particular site, a gardener or landscape designers must understand whether or not a give species is likely to thrive in the environment presented by that site or microclimate within the site. Most horticulturists are familiar with the USDA Plant Hardiness Zone Map, and assume that a plant will thrive in their area if it is listed for their zone. Unfortunately, this is NOT true, especially in Colorado and the Rocky Mountain West.

The USDA Plant Hardiness Zone Map was developed in the 1960’s by the National Arboretum, and revised in the 1990’s. Its sole purpose is to help establish where plants were likely to survive the average minimum winter temperature and still thrive if given all the right “other conditions”. The map divides the United States into 11 zones using intervals of 10°F change in the minimum winter temperature. A plant’s listing for a particular zone is determined by the observations the National Arboretum has collected from horticulturists across the nation. This system works well for estimating winter survival in the Southeastern, Eastern, and Midwestern United States, where the land is relatively flat and the winter weather patterns are relatively predictable. Unfortunately, the system breaks down in the Western United states where the affects of elevation, exposure (wind, and precipitation patterns, etc), humidity, soil, and aspect (north, south, east or west, sunny or shaded locations) all compound to create drastically different microclimates that will affect a individual plant’s growing conditions and thus its ability to thrive at a given site. Anyone who has gardened or hiked extensively in Colorado has probably already noted that plant, especially trees, form distinct communities in distinct habitats. These observably different habitats are often referred to as microclimates by gardeners. These microclimates are usually a distinct combination of a soil, a specific exposure, and specific moisture and temperature regimes.

Many Colorado horticulturists use the USDA zones to gauge the likelihood of a plant surviving the coldest winter in their area of Colorado. They may also consider their elevation, and the elevation the plant has been recorded as growing to. THEORETICALLY, each 1000-foot increase in elevation shortens the growing season by about one-month and is approximately equal to a drop of one hardiness zone. While this system offers a slightly better prediction of plant adaptability, it too is unreliable (see table 1 for examples) because of the affects of microclimatic differences. Ones ability to decide whether or not a plant is hardy enough for a particular situation depends a lot on ones own experience with the species or related plants, and ones ability to observe what environmental factors in a particular situation lead to a plants’ success (and vigor), or lack of vigor and eventual failure. Briefly, here are some factors to consider when deciding why a plant failed or “wasn’t hardy”.

  • What sort of care did the plant receive?
  • How well did the conditions of a site match that plant’s natural habitat?
  • How well did the plant’s annual growth cycle (phenology) match the conditions of climate and microclimate in which it was forced to grow?
  • What about soil type, fertility and moisture conditions thought out the year? Did these create disease problems that would be avoided with another exposure or different care?

Finding plants whose seasonal growth cycles match a particular site’s growing season (first and last frost dates and average number of frost free days) is the really first key to creating a successful landscape. Plants out of sync with the natural growing season are vulnerable to late spring or early fall freezes, breakage during unseasonable storms, and winterkill. Although native plants are often less prone to damage from such storms (they’ve had millions of years of evolutionary adaptation to put them in sync with the extremes of our climate) there are many introduced species that are equally resistant to our extreme weather. The trick though is finding them.

The soil a plant is growing in will affect its hardiness in numerous ways. The soil’s parent material, fertility, and pH will determine whether or not the plant has the nutrient resources it needs for proper growth. The presence or absence of soil mycorrhizae or soil borne pathogens will affect the health of the root system, and soil texture and compaction will affect the aeration which may limit root growth or encourage pathogen growth. For trees, proper planting technique and proper soil care after planting will make “THE” difference in the trees establishment and longevity. The best source of information on proper planting techniques and care for tree is the International Society of Arboriculture, and the publications of the numerous State Land Grant Universities.

The care a plant receives may also dramatically affect its survival and performance. Especially important to a tree’s survival is the pattern and amount of moisture (irrigation) and fertility it receives. Excessive irrigation will always limit root growth, by reducing aeration. Excessive late season moisture, especially when combined with excessive fertilization, can also counteract a plant’s natural dormancy mechanisms delaying the onset of its winter dormancy, or spurring abnormal fall growth. Such conditions will cause the plant to hold its leaves into late fall, making it plant vulnerable to freeze or storm damage. Excessive growth caused by late season irrigation and fertilization may also reduce a tree’s winter food stores making it more susceptible to winter kill or pest infestations.

Equally important to the winter survival of a plant is its ability to withstand attacks of pests or diseases. Small populations of pests or mild disease attacks may not be a major concern to horticulturists; as plants are usually adapted to supporting small pest populations. Large or sever pest and diseases outbreaks may kill trees outright, or may weaken them severely, by reducing their stored carbohydrate levels to the point that they cannot recover sufficiently or in a timely manner to store sufficient carbohydrates to survive the winter.